1. Field of the Invention
The present invention relates to a communication method in a wireless communication system, and more particularly, to a method for transmitting a downlink reference signal in a multi-antenna wireless communication system.
2. Discussion of the Related Art
3rd Generation Partnership Project (3GPP) wireless communication systems based on Wideband Code Division Multiple Access (WCDMA) radio access technology have been deployed widely all over the world. The first evolution of WCDMA, High Speed Downlink Packet Access (HSDPA) provides the 3GPP with a highly competitive radio access technology in a mid-term perspective.
Evolved Universal Mobile Telecommunication System (E-UMTS) was designed to offer high competitiveness in a long term perspective. The 3GPP is working on the standardization of E-UMTS, an evolution of WCDMA UMTS. E-UMTS is also called Long Term Evolution (LTE). For details of the technical specifications of UMTS and E-UMTS, refer to Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”
The E-UMTS system largely includes User Equipments (UEs), Base Stations (BSs) (Node Bs or evolved Node Bs (eNBs)), and an Access Gateway (AG) located at an end of an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and connected to an external network. In general, a BS may transmit multiple data streams simultaneously for a broadcast service, a multicast service, and/or a unicast service. The LTE system employs Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) to provide a variety of services on a downlink.
OFDM, which is a major HSDPA system, offers the benefit of high spectral efficiency since an allocated entire spectrum is available to all BSs. For OFDM modulation, a transmission band is divided into a plurality of orthogonal subcarriers in the frequency domain and a plurality of symbols in the time domain. Due to the division of the transmission band into a plurality of subcarriers, OFDM is characterized by a decreased bandwidth per subcarrier and an increased modulation time perspective view carrier. Because the plurality of subcarriers is transmitted in parallel, the transmission rate of digital data or symbols on a specific subcarrier is lower than a single carrier.
MIMO is a communication system using a plurality of Transmission (Tx) antennas and a plurality of Reception (Rx) antennas. The MIMO system may increase channel capacity linearly with the number of Tx and Rx antennas, without an additional increase in frequency bandwidth. There are two types of MIMO schemes, spatial diversity and spatial multiplexing. Spatial diversity increases transmission reliability by transmitting symbols in a plurality of channel paths, while spatial multiplexing increases transmission rate by transmitting different data streams simultaneously through a plurality of Tx antennas.
MIMO schemes may also be classified into open-loop MIMO and closed-loop MIMO depending on whether a transmitter has knowledge of channel information. Open-loop MIMO does not require that the transmitter is aware of channel information. Open-loop MIMO schemes include Per Antenna Rate Control (PARC), Per Common Basis Rate Control (PCBRC), vertical Bell Labs layered Space Time (BLAST), Space Time Trellis Code (STTC), random beamforming, etc. In contrast, the transmitter has channel information in closed-loop MIMO. The performance of a closed-loop MIMO system depends on how accurate channel information the transmitter gets. Closed-loop MIMO schemes include Per Stream Rate Control (PSRC), Transmit Adaptive Array (TxAA), etc.
Channel information is information about radio channels between a plurality of Tx antennas and a plurality of Rx antennas (e.g. attenuation, a phase shift, a time delay, etc.). Many stream paths exists according to the combinations of the Tx and Rx antennas and channel status fluctuates over time in the time and frequency domains in view of a multipath time delay, which is called fading, in the MIMO system. Accordingly, the transmitter calculates the channel information through channel estimation. Channel estimation is the process of estimating channel information required for recovering a distorted transmission signal. For example, the channel estimation is equivalent to estimation of the amplitude and reference phase of a carrier. In other words, the channel estimation is to estimate the frequency response of a radio interface or a radio channel.
For channel estimation, a reference value may be estimated from several Reference Signals (RSs) received from a BS using a two-dimensional channel estimator. An RS is symbols transmitted at a high power level without carrying actual data to help carrier phase synchronization and BS information acquisition. Both the transmitter and the receiver may perform channel estimation using RSs. Specifically, the RS-based channel estimation is to estimate a channel using symbols known to both the transmitter and the receiver and recover data based on the channel estimate. An RS is called a pilot signal.
The MIMO system supports Time Division Duplex (TDD) and Frequency Division Duplex (FDD). In TDD, downlink transmission and uplink transmission take place in the same frequency area, thereby making it possible to estimate a downlink channel from an uplink channel based on a reciprocity principle.
The WCDMA-based wireless communication technology has evolved to LTE. Nevertheless, the demands and expectations of users and service providers are on the increase. Considering other wireless access technologies are under development, a further technological evolution is required to achieve future competitiveness. The requirements of the new technological evolution include the reduction of cost per bit, the increase of service availability, flexible use of frequency bands, simplified structures and open interfaces, appropriate power consumption of UEs, etc.
In this context, the 3GPP is preparing for standardization of a successor to LTE, so-called LTE-Advanced (LTE-A). One of major differences between LTE and LTE-A is the number of supported multiple antennas. While a legacy LTE system supports up to four antennas, whereas an LTE-A system aims to support up to eight antennas. Therefore, the LTE-A system should be able to support downlink transmission of RSs through up to eight antennas. Especially in the LTE-A system, an LTE terminal able to recognizing up to four antennas may coexist with an LTE-A terminal able to recognizing up to eight antennas.